This invention is related to a heat engine and particularly to an improved Stirling cycle engine incorporating numerous refinements and design features intended to enhance engine performance, manufacturability, and reliability.
The basic concept of a Stirling engine dates back to a patent registered by Robert Stirling in 1817. Since that time, this engine has been the subject of intense scrutiny and evaluation. Various Stirling engine systems have been prototyped and put into limited operation throughout the world. One potential application area for Stirling engines is for automobiles as prime mover or engine power units for hybrid electric applications. Such applications place extreme demands on Stirling engine design. Due to the wide acceptance of spark ignition and Diesel engines, to gain acceptance, a Stirling engine must show significant advantages over those types, such as a dramatic enhancement in fuel efficiency or other advantages. In addition, reliability and the ability to manufacture such an engine at a low cost are of paramount importance in automotive applications. Similar demands are present in other fields of potential use of a Stirling engine such as stationary auxiliary power units, marine applications, solar energy conversion, etc.
Stirling engines have a reversible thermodynamic cycle and therefore can be used as a means of delivering mechanical output energy from a source of heat, or acting as a heat pump through the application of mechanical input energy. Using various heat sources such as combusted fossil fuels or concentrated solar energy, mechanical energy can be delivered by the engine. This energy can be used to generate electricity or be directly mechanically coupled to a load. In the case of a motor vehicle application, a Stirling engine could be used to directly drive traction wheels of the vehicle through a mechanical transmission. Another application in the automotive environmental is for use with a so-called "hybrid" vehicle in which the engine drives an alternator for generating electricity which charges storage batteries. The batteries drive the vehicle through electric motors coupled to the traction wheels. Perhaps other technologies for energy storage could be coupled to a Stirling engine in a hybrid vehicle such as flywheel or thermal storage systems, etc.
The Assignee of the present application, Stirling Thermal Motors, Inc. has made significant advances in the technology of Stirling machines for a number of years. Examples of such innovations include the development of a compact and efficient basic Stirling machine configuration employing a parallel cluster of double acting cylinders which are coupled mechanically through a rotating swashplate. In many applications, a swashplate actuator is implemented to enable the swashplate angle and therefore the piston stroke to be changed in accordance with operating requirements.
Although the Assignee has achieved significant advances in Stirling machine design, there is a constant need to further refine the machine, particularly if the intended application is in large volume production. For such applications, for example in motor vehicles, great demands are placed on reliability and cost. It is well known that motor vehicle manufacturers around the world have made great strides in improving the reliability of their products. The importance of a vehicle engine continuing to operate reliably cannot be overstated. If a Stirling engine is to be seriously considered for motor vehicle applications, it must be cost competitive with other power plant technologies. This is a significant consideration given the mature technology of the spark ignition and Diesel internal combustion engines now predominately found in motor vehicles today.
During the past several decades, significant improvements in exhaust pollution and fuel economy have been made for spark ignition and Diesel engines. However, there are fundamental limits to the improvements achievable for these types of internal combustion engines. Due to the high temperature intermittent combustion process which takes place in internal combustion engines, pollutants are a significant problem. Particularly significant are NO.sub.x and CO emissions. Although catalytic converters, engine control, and exhaust treatment technologies significantly improve the quality of emissions, there remains room for improvement. Fuel efficiency is another area of concern for the future of motor vehicles which will require that alternative technologies be studied seriously. It is expected that the ultimate thermal efficiency achievable with the spark ignition internal combustion engines is on the order of 20%, with Diesel engines marginally exceeding this value. However, in the case of Stirling engines, particularly if advanced ceramic or other high temperature materials are implemented, thermal efficiencies in the neighborhood of 40% to 50% appear achievable. The external combustion process which could be implemented in an automotive Stirling engine would provide a steady state combustion process which allows precise control and clean combustion. Such a combustion system allows undesirable pollutants to be reduced.
In view of the foregoing, there is a need to provide a Stirling cycle engine having design features enabling it to be a viable candidate for incorporation into large scale mass production such as for automobiles and for other applications. The present invention relates to features for a Stirling engine which achieves these objects and goals.
The Stirling engine in accordance with the present invention has a so-called "modular" construction. The major components of the engine, comprising the drive case and cylinder block, are bolted together along planar mating surfaces. Piston rod seals for the pistons traverse this mating plane. A sliding rod seal can be used which is mounted either to the drive case or cylinder block. The rod seal controls leakage of the high pressure engine working gas at one end of the rod to atmosphere. Sliding contact rod seals provide adequate sealing for many applications. For example, in an automotive engine such an approach might be used. The sliding contact seal would, however, inevitably allow some leakage of working fluid, if only on a molecular level. In solar energy conversion or other applications where the engine must operate over an extremely long life, other types of sealing technology may be necessary to provide a hermetic, i.e., non-leaking, seal. In the engine of this invention, if other rod sealing approaches are required, it would be a simple matter to insert a plate between the drive case and cylinder block which supports a bellows or other type of hermetic sealing element. Thus the same basic engine componentry could be implemented for various applications.
The Stirling engine of the present invention further includes a number of features which enable it to be manufactured efficiently in terms of component costs, processing, and parts assembly. The drive case and cylinder block feature a number of bores and passageways which can be machined at 90.degree. from their major mounting face surfaces, thus simplifying machining processes. Designs which require castings to be machined at multiple compound angles and with intersecting passageways place more demands on production machinery, tools, and operators, and therefore negatively impact cost.
The Stirling engine according to this invention provides a number of features intended to enhance its ease of assembly. An example of such a feature is the use of a flat top retaining plate which mounts the cylinder extensions and regenerator housings of the engine in place on the cylinder block. The use of such flat surfaces and a single piece retaining plate simplifies machining and assembly. The retaining plate design further lowers cost by allowing a reduction in the high temperature alloy content of the engine. Furthermore, the one-piece retaining plate provides superior component retention as compared with separate retainers for each cylinder extension and regenerator housing.
In many past designs of Stirling engines, a large volume of the engine housing is exposed to the high working pressures of the working gas. For example, in many of the Assignee's prior designs, the entire drive case was subject to such pressures. For such designs, the entire housing might be considered a "pressure vessel" by certifying organizations and others critically evaluating the engine from the perspective of safety concerns. Thus, the burst strength of the housing may need to be dramatically increased. This consideration would greatly increase the cost, weight, and size of the machine. In accordance with the engine of the present invention, the high pressure working fluid is confined to the extent possible to the opposing ends of the cylinder bores and the associated heat transfer devices and passageways. Thus the high pressure gas areas of the Stirling engine of this invention are analogous to that which is encountered in internal combustion engines, and therefore this Stirling engine can be thought of in a similar manner in terms of consideration for high pressure component failure. This benefit is achieved in the present invention by maintaining the drive case at a relatively low pressure which may be close to ambient pressure, while confining the high pressure working fluid within the cylinder block and the connected components including the cylinder extension, regenerator housing, and heater head.
As a means of enhancing the degree of control of operation of the Stirling engine of this invention, a variable piston stroke feature is provided. In order to achieve this, some means of adjusting the swashplate angle is required. In many past designs, hydraulic actuators were used. These devices, however, consume significant amounts of energy since they are always activated and tend to be costly to build and operate. This invention encompasses two versions of electric swashplate actuators. A first version features a rotating motor which couples to the swashplate drive through a planetary gear set. A second embodiment incorporates a stationary mounted motor which drives the actuator through a worm gear coupled to a pair of planetary gear sets. In both cases, a high gear reduction is achieved, which through friction in the mechanically coupled element, prevents the actuator from being back-driven and thus a swashplate angle can be maintained at a set position without continuously energizing the drive motor. Power is applied to the drive motor only when there is a need to change the swashplate angle and hence piston stroke.
The pistons of the engine are connected to cross heads by piston rods. The cross heads of the engine embrace the swashplate and convert the reciprocating movement of the piston connecting rods and pistons to rotation of the swashplate. The Stirling engine of this invention implements a pair of parallel guide rods mounted within the drive case for each cross head. The cross heads feature a pair of journals which receive the guide rods.
The cross heads include sliders which engage both sides of the swashplate. The clearance between the sliders and the swashplate surfaces is very critical in order to develop the appropriate hydro-dynamic lubricant film at their interfaces. An innovative approach to providing a means of adjusting the cross head bearing clearances is provided in accordance with the present invention.
This invention further encompasses features of the piston assemblies which include a sealing approach which implements easily machined elements which provide piston sealing. A pair of sealing rings are used and they are subjected to fluid forces such that only one of the sealing rings is effective in a particular direction of reciprocation of the piston. This approach reduces friction, provides long ring life and enhances sealing performance.
The combustion exhaust gases after passing through the heater head of the engine still contain useful heat. It is well known to use an air preheater to use this additional heat to heat incoming combustion air as a means of enhancing thermal efficiency. In accordance with this invention, an air preheater is described which provides a compact configuration with excellent thermal efficiency. The surfaces of the preheater exposed to combustion gases can be coated with a catalyst material such as platinum, palladium or other elements or compounds which enable the combustion process to be further completed, thus generating additional thermal energy. The catalyst further reduces exhaust emissions as they do for today's internal combustion engines.
The Stirling engine of this invention incorporates a heater head assembly with a number of tubes which are exposed to combustion gases enabling the heat of combustion to be transferred to the working gas within the engine. The typical approach toward constructing such a heater head assembly is to painstakingly bend tubing to the proper configuration with each tube having a unique shape. Such an approach is ill-suited for volume production. The requirement of using bent tubing also places significant limitations on heater head performance. Material selections are limited since it must have adequate ductility to enable tube stock formed in straight runs or coils to be bent to the proper shape. Such tubing also has a uniform wall thickness and cannot readily be incorporated with external fins to enhance heat transfer area without welding or brazing additional parts to the outside of the tube. These steps add to cost and complexity. Moreover, when brazing materials are used, temperature limits are placed on the heater tubes to avoid failure of these joints. This temperature limitation also reduces thermal efficiency which tends to increase with combustion temperature. In accordance with this invention, cast heater tubes are provided which can be made in multiples of the same configuration connected together through a heater head. The cast material allows the heater tubes to be subjected to much higher temperatures. In addition, special configurations can be provided to enhance performance. For example, fins of various cross-sectional shape can be provided. Also, the fins need not have a rotationally symmetric configuration, but instead can be designed to consider the fluid mechanics of the fluids moving across them. Through appropriate fin design, it is believed possible to cause the entire perimeter of the heater tubes to be a near uniform temperature despite the fact that fluids are flowing transversely across them. Temperature gradients associated with prior heater tube designs place significant thermal stresses on the tubes, which over time, lead to mechanical fatigue failure.
In the Stirling engine of the type according to the present invention employing four double acting cylinders, there are four discrete volumes of working gas which are isolated from one another (except by leakage across the pistons). In order to enable the engine to operate smoothly and with minimal force imbalances, the mean pressure of each of these four volumes need to be equalized. In accordance with this invention, this is achieved by connecting together the four volumes through capillary tubes. In addition, a system is provided for determining that the mean pressure in each cycle is within a predetermined range. Upon the occurrence of a component failure causing leakage, a significant imbalance could result which could have a destructive effect on the engine. The Stirling engine according to this invention features a pressure control system which unloads the engine upon the occurrence of such failure.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.